1. Field of the Invention
The present invention relates to a method and apparatus for processing video data of a display device, and more particularly, to a method and apparatus for processing video data of a display device in which dithering noise generating when a motion picture is displayed can be minimized.
2. Description of the Background Art
A variety of display devices such as a liquid crystal display panel, an electro luminescence panel and a plasma display panel has been developed and employed. Of them, the plasma display panel (hereinafter, referred to as a ‘PDP’) is adapted to display an image by using a visible ray generating from phosphors when ultraviolet generated by a gas discharge excite the phosphors. This PDP is advantageous it that it can provide the slimness, the compact size, higher definition and large screen, compared to a cathode ray tube (CRT) which has become the main stream of the display device so far.
FIG. 1 is a plan view schematically illustrating a conventional PDP. FIG. 2 is a perspective view illustrating the structure of the cell shown in FIG. 1.
Referring to FIGS. 1 and 2, the three-electrode AC surface discharge type PDP includes scan electrodes Y1 to Yn and sustain electrodes Z which are formed on the bottom surface of an upper substrate 10, and address electrodes X1 to Xm formed on the top of a lower substrate 18.
The discharge cells 1 of the PDP are formed at respective crossings of the scan electrodes Y1 to Yn, the sustain electrodes Z and the address electrodes X1 to Xm.
Each of the scan electrodes Y1 to Yn and the sustain electrodes Z includes a transparent electrode 12, and a metal bus electrode 11, which has a line width narrower than that of the transparent electrode 12 and is disposed at one edge side of the transparent electrode. The transparent electrode 12, which is generally made of ITO (indium tin oxide), is formed on the bottom surface of the upper substrate 10. The metal bus electrode, which is typically made of metal, is formed on the transparent electrode 12, and serves to reduce a voltage drop caused by the transparent electrode 12 having high resistance. On the bottom surface of the upper substrate 10 in which the scan electrodes Y1 to Yn and the sustain electrodes Z are formed is laminated an upper dielectric layer 13 and a protective layer 14. The upper dielectric layer 13 is accumulated with wall charges generated during plasma discharge. The protective layer 14 serves to prevent damage of the electrodes Y1 to Yn and Z and the upper dielectric layer 13 due to sputtering generated by the plasma discharge, and improve emission efficiency of secondary electrons. Magnesium oxide (MgO) is generally employed as the protective layer 14.
The address electrodes X1 to Xm are formed on the lower substrate 18 in a direction in which they cross the scan electrodes Y1 to Yn and the sustain electrodes Z. A lower dielectric layer 17 and barrier ribs 15 are formed on the lower substrate 18. The barrier ribs 15 are formed in the form of a stripe or grating to physically separate the discharge cells 1, thus prohibiting electrical and optical interference among discharge cells 1. The phosphor layer 16 is excited and light-emitted with ultraviolet generating during the plasma discharge to generate any one of red, green and blue visible rays.
An inert mixed gas such as He+Xe, Ne+Xe or He+Ne+Xe is injected into the discharge spaces of the discharge cells defined between the upper substrate 10 and the barrier ribs 15 and between the lower substrate 18 and the barrier ribs 15.
This PDP is driven with one frame being divided into a plurality of sub-fields having a different number of emission in order to implement the gray scale of an image. Each of the sub fields is divided into a reset period for uniformly generating a discharge, an address period for selecting a discharge cell, and a sustain period for implementing the gray level according to the number of a discharge. For example, if it is desired to display an image with 256 gray scales, a frame period (16.67 ms) corresponding to 1/60 second is divided into eight sub-fields. Each of the eight sub-fields is subdivided into the reset period, the address period and the sustain period. The reset period and the address period of each of the sub-fields are the same every sub-field, whereas the sustain period and the number of the discharge increase in the ratio of 2n (where, n=0,1,2,3,4,5,6,7) in each sub-field. As such, since the sustain periods become different in the respective sub-fields, the gray scale of an image can be implemented.
FIG. 3 is a block diagram of an apparatus for driving a PDP in a prior art.
Referring to FIG. 3, the apparatus for driving the PDP includes a gamma correction unit 30, an error diffusion unit 32, a dithering unit 34, a sub-field mapping unit 36 and a data driving unit 38 all of which are connected between an input line of video data and a panel 40.
The gamma correction unit 30 performs an inverse gamma correction operation on digital video data, which has undergone a gamma correction operation so that the data is suitable for a brightness characteristic of a cathode ray tube (CRT). In this time, the digital video data, which has undergone the inverse gamma correction operation by the gamma correction unit 30, has a linear brightness characteristic.
The error diffusion unit 32 diffuses error of the video data from the gamma correction unit 30 and pixels, which are calculated through an error diffusion filter, by using error diffusion coefficients. For example, as shown in FIG. 4, if the error diffusion operation is performed on a current pixel P5, error is diffused by assigning a weight of 1/16 to a pixel P1 adjacent to the pixel P5, a weight of 5/16 to a pixel P2, a weight of 3/16 to a pixel P3 and a weight of 7/16 to a pixel P4.
The dithering unit 34 expands the gray scale by selecting discharge cells to be turned on, through a dithering method that employs dither mask patterns. As such, if the gray scale is expanded through selection of the discharge cells to be turned on by means of the dithering method, contour noise can be removed. For example, European Patent Application No. 00250099.9, etc. discloses a method of selecting discharge cells to be turned on by using three-dimensional dither mask patterns corresponding to a plurality of frames, a plurality of lines and a plurality of columns in a plasma display panel.
The sub-field mapping unit 36 maps each of the pixel data from the dithering unit 34 to a predetermined sub-field pattern, and outputs the mapped results.
The data driving unit 38 latches the data, which is separated on a bit basis according to the sub-field patterns by the sub-field mapping unit 36, and supplies the latched data to address electrode lines of the panel 40 for one line every period where a horizontal line is driven.
Te panel 40 displays a given image corresponding to data, which is supplied from the data driving unit 38 to the address electrode lines.
Such a conventional PDP has a problem in that dithering noise is generated when a motion picture is displayed. This will be described in detail as follows. The dithering unit 34 employs dither mask patterns, which are different every frame. In reality, the dithering unit 34 uses dither mask patterns, which are repeated in an approximately two to four frame unit. In this time, dither values 1 of the dither mask patterns, which have the same gray scale and are also used in an ith (i is a natural number) frame and a (i+1)th frame, are disposed at different positions (e.g., grating patterns). For example, the dither mask patterns used in the ith frame and the (i+1)th frame can be set, as shown in FIG. 5.
In the case where a still image is displayed, if the still image is combined with the ith frame and the (i+1)th frame, discharge cells to be turned are selected in the form of gratings, as shown in FIG. 6. As such, if the discharge cells to be turned on are selected in the form of gratings, an image of the picture quality with no noise can be displayed. However, if a motion picture where the (i+1)th frame is moved by one pixel is to be displayed, dither mask patterns as shown in FIG. 7 are recognized to the eye of a man in the form of stripes. As such, if the discharge cells to be turned on are recognized in the form of stripes, noise of a specific shape is generated in an image displayed on the panel 40, which makes the eye of a man unpleasant.